I. Field of the Invention
The invention relates generally to wireless communications. More particularly, the invention relates to handoff control in a wireless communication system.
II. Description of the Relaxed Art
FIG. 1 is an exemplifying embodiment of a terrestrial wireless communication system 10. FIG. 1 shows the three remote units 12A, 12B and 12C and two base stations 14. In reality, typical wireless communication systems may have many more remote units and base stations. In FIG. 1, the remote unit 12A is shown as a mobile telephone unit installed in a car. FIG. 1 also shows a portable computer remote unit 12B and the fixed location remote unit 12C such as might be found in a wireless local loop or meter reading system. In the most general embodiment, remote units may be any type of communication units. For example, the remote units can be hand-held personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link signal 18 from the base stations 14 to the remote units 12 and a reverse link signal 20 from the remote units 12 to the base stations 14.
An industry standard for a wireless system using code division multiple access (CDMA) is set forth in the TAU/EIA Interim Standard entitled xe2x80x9cMobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, TIA/ELA/IS-95, and its progeny (collectively referred to herein as IS-95), the contents of which are also incorporated herein by reference. More information concerning a code division multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention and incorporated in its entirety herein by this reference.
In an IS-95 system, each base station synchronizes its operation with other base stations in the system. For example, in one embodiment, the IS-95 base stations synchronize operation to a universal time reference such as Global Positioning Satellites (GPS) signaling. Based upon the synchronizing time reference, each base station in a given geographical area is assigned a sequence offset of a common pseudo noise (PN) pilot sequence. For example, according to IS-95, a PN sequence having 215 chips and repeating every 26.66 milliseconds (ms) is transmitted by each base station in the system at one of 512 PN sequence offsets as a pilot signal. According to IS-95 operation, the base stations continually transmit the pilot signal which can be used by the remote units to identify the base stations as well as for other functions.
Various methods exist for transferring communication with the remote unit from one base station to another through a process known as handoff. Handoff may be necessary if a remote unit operating in the coverage area of an original base station moves into the coverage area of a target base station. One method of handoff used in CDMA systems is termed a xe2x80x9csoftxe2x80x9d handoff. Through the use of soft handoff, communication with the target base station is established before termination of communication with the original base station. When the remote unit is communicating with two base stations, both the remote unit and base stations create a single signal from the multiple received signals. Through the use of soft handoff, communication between the remote unit and the end user is uninterrupted by the eventual handoff from the original base station to the target base station. U.S. Pat. No. 5,267,261 entitled xe2x80x9cMOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM,xe2x80x9d which is assigned to the assignee of the present invention and which is incorporated herein, discloses a method and system for providing communication with the remote unit through more than one base station during the handoff process. Further information concerning handoff is disclosed in U.S. Pat. No. 5,101,501, entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, U.S. Pat. No. 5,640,414, entitled xe2x80x9cMOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEMxe2x80x9d, and U.S. Pat. No. 5,625,876 entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION,xe2x80x9d each of which is assigned to the assignee of the present invention and incorporated in its entirety herein by this reference. The subject matter of U.S. Pat. No. 5,625,876 concerns so-called xe2x80x9csofter handoff.xe2x80x9d For the purposes of this document, the term xe2x80x9csoft handoffxe2x80x9d is intended to include both xe2x80x9csoft handoffxe2x80x9d and xe2x80x9csofter handoff.xe2x80x9d
As described in the above mentioned patents, remote unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the remote unit: the active set, the neighbor set, the candidate set and remaining set. The active set is the set of base stations through which active communication is established. The neighbor set is a set of base stations surrounding the active base stations and comprising base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication. The candidate set is a set of base stations having a pilot signal strength of sufficient level to establish communication but through which active communication is not yet established. The remaining set is a set of base stations which are not in any of the other three sets.
The remote unit uses these sets to control the handoff process. In this example, we shall assume that when communications are initially established, a remote unit communicates through a first base station, and the active set contains only the first base station; although in many cases, the active set contains more than one base station before the handoff process is begun with respect to yet another base station. The remote unit monitors the pilot signal strength of the base stations in the active set, the candidate set, the neighbor set and the remaining set. When a pilot signal strength of a base station in the neighbor or remaining set exceeds a predetermined threshold level, the base station is added to the candidate set and removed from the neighbor or remaining set at the remote unit. The remote unit communicates a pilot strength measurement overhead message through the first base station identifying the new base station. A system controller receives the pilot strength measurement overhead message from the first base station and decides whether to establish communication between the new base station and the remote unit. Should the system controller decide to do so, the system controller sends a message to the new base station with identifying information about the remote unit and a command to establish communications with the remote unit.
A handoff message is also transmitted to the remote unit through the first base station. The handoff message is an overhead message which identifies a new active set that includes the first and the new base stations. The handoff message also identifies which channel has been allocated for use by the remote unit with the new base station. The remote unit searches for the new base station""s transmitted signal, and communication is established with the new base station without termination of communication through the first base station. This process can continue with additional base stations such that two or more base stations are in the active set.
When the remote unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the active set, the candidate set, the neighbor set and the remaining set. Should the signal strength corresponding to a base station of the active set drop below a predetermined threshold for a predetermined period of time, the remote unit generates and transmits an overhead message to report the event. The system controller receives this message through at least one of the base stations with which the remote unit is communicating. In response to receiving this message, the system controller can decide to terminate communications through the base station having a weak signal strength.
Upon forming a decision to terminate communications through a base station, the system controller generates a handoff message identifying a new active set of base stations. The new active set does not contain the base station through which communication is to be terminated. The base stations through which communication is established send the handoff message to the remote unit with the new active set. The remote unit receives the overhead message and removes the base station from the active set and, typically, places it in the neighbor set. The remote unit communications are, thus, routed only through base stations identified in the new active set.
Because the remote unit is communicating with the end user through at least one base station at all times throughout the soft handoff process, no interruption in communications occurs between the remote unit and the end user. A soft handoff provides significant benefits in its inherent xe2x80x9cmake before breakxe2x80x9d communication over conventional xe2x80x9cbreak before makexe2x80x9d (hard handoff) techniques employed in cellular communication systems employing other multiple access techniques such as time division multiple access (TDMA) or frequency modulation (FM).
As noted above, each base station is associated with a set of neighboring base stations surrounding the base station. Due to the close physical proximity of the coverage areas of the neighboring base stations to the coverage area of the active base station, the remote units which are communicating with the active base station are more likely to handoff to one of the neighboring base stations than to other base stations in the system. The base station identifies the neighboring base stations to the remote units with which it has established communication using a neighbor list identification message. The neighbor list identification message identifies a neighboring base station according to the PN sequence offset at which it transmits the pilot signal.
Due to path delays and multipath, the relative time offset between two pilot signals arriving at a remote unit from neighboring base stations is typically not identically equal to the nominal PN sequence offset. In addition, the delay and the multipath environment is constantly changing due to the relative movement of objects within the base station coverage areas. Therefore, a searching element within the remote unit is used to search for the pilot signals of the neighboring base stations over a range of PN sequence offsets relative to known timing conditions.
Each search which the searching element performs can be characterized as having a nominal PN sequence offset and a corresponding search window. The search window specifies a set of PN sequence offsets around the nominal PN sequence offset. Generally, the search window comprises a range of offsets in which the remote unit is likely to detect a pilot signal. For each offset processed, the searching element finds the correlation energy at that offset by despreading the antenna samples using the same PN sequence used to generate the pilot signal. The searching element accumulates the energy for a period of time and reports the accumulated energy to a remote unit controller. If the accumulated energy exceeds a certain threshold, a pilot signal is detected.
The remote unit uses the neighbor list to limit the space over which it searches for handoff candidates. For example, because the searching process is so resource intensive, it is advantageous to avoid performing a search over the entire set of possible PN sequence offsets. By using the neighbor list, the remote unit can concentrate its resources on those PN sequence offsets which are most likely to correspond to useful signal paths.
The IS-95 operation is practical so long as each base station""s timing remains synchronous with respect to the others. However, in some systems, advantages are achieved by decoupling operation of the system from a synchronizing reference. For example, in a system which is deployed underground, such as in a subway system, it can be difficult to derive a universal time synchronization signal using GPS. In addition, in certain political climates, it is perceived as desirable to decouple system operation from signaling under the control of another political entity.
In a system where one or more of the base stations operate asynchronously with respect to other base stations in the system, the base stations cannot be distinguished from one another based upon a relative time offset (typically measured as a relative PN sequence offset) because a relative time offset between the base stations cannot be established without the use of a universal time reference. Therefore, the handoff control systems just described must be modified to accommodate asynchronous operation.
Thus, there is a need in the art to develop a handoff control mechanism for use in an asynchronous CDMA system.
In one embodiment, in a system where a wireless remote unit is capable of communication with multiple base stations simultaneously, handoff to a subsequent base station is controlled through use of a selected set of base stations. For example, if frame synchronization between a second base station and the active base station is unknown, when a second base station has signal strength sufficient to establish communication, the remote unit transmits a message to a network controller identifying the second base station. The network controller determines an availability of resources at the second base station. If resources are available, the base station sends a message via the first base station identifying the second base station as a selected base station. In response, the remote unit monitors an overhead channel from the second base station in order to determine a frame synchronization of the second base station. The remote unit transmits the frame synchronization to the network controller. The network controller commands the second base station to begin transmission of signals to the remote unit such that the transmission from the second base station arrives at the remote unit approximately synchronized with the transmission from the first base station.
In another embodiment, time alignment in a wireless communications system is achieved by the remote unit without using the round trip messages of the selected set embodiment. For example, the remote unit receives a forward link transmission from a first base station having a first frame alignment. The remote unit receives a second forward link transmission from a second base station having a second frame alignment. The second frame alignment comprises information concerning frame boundaries but not information concerning an absolute frame number. The remote unit combines the first forward link transmission and the second forward link transmission according to a first frame alignment. The remote unit determines whether a performance indication is within expected limits. If the performance indication is not within the expected limits, the remote unit combines the first forward link transmission and the second forward link transmission using a second frame alignment.
In yet another embodiment, a composite neighbor list is created by the remote unit or the network controller for use during the handoff process. For example, in the case of the remote unit determined composite neighbor list, the remote unit receives one or more neighbor lists corresponding to one or more active base stations. The remote unit removes entries from the one or more neighbor lists corresponding to base stations through which active communication is established. The remote unit aligns a time offset reference of at least one entry in the neighbor lists so that the entries in the neighbor lists are referenced to a common timing reference. For each base station having more than one entry on the neighbor list, the remote unit determines a single composite entry specifying a composite search window equal to the intersection of a search window specified in each entry corresponding to the base station.
In yet a further embodiment, the neighbor list used to facilitate handoff comprises a series of entries. The entries comprise information identifying a reference base station and a PN offset. A timing of the reference base station is used as a reference timing for the PN offset. The entries may further comprise a window size over which a search should be performed. The window size may implicitly carry information concerning a relative class of synchronization between the reference base station and the base station to which the entries correspond. For example, the window size may carry information concerning whether the base station corresponding to the entry is frame synchronized with the reference base station.